Dot-etchable photopolymerizable elements

ABSTRACT

High contrast, dot-etchable solvent-processable lithographic film elements are described comprising a sheet support, a photopolymerizable layer having a thickness of, at most, 0.0004 inch (0.010 mm) and an optical density of at least 3.0 in the actinic region. Images made from these elements may be reduced in area by a process of chemically undercutting the image areas and then spraying or rubbing them. The elements are useful as contact speed lithographic films and for other graphic arts applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 802,512, filedJune 1, 1977, now abandoned, which is a division of application Ser. No.741,039, filed Nov. 11, 1976, now abandoned which is acontinuation-in-part of application Ser. No. 632,726, filed Nov. 17,1975, now abandoned.

This invention relates to new photopolymerizable elements which can beused for preparing novel, dot-etchable image-containing masks that areuseful for copy preparation and related purposes. Masks are used inimagewise exposing other photosensitive elements with actinic radiation.

In the photomechanical trades, a mask containing an image that is opaqueto actinic radiation is used in preparing a printing plate of some kind;whether letterpress or lithographic, the method of production is muchthe same. A metal (or in some cases, plastic) plate is coated with aphotosensitive resist-forming material, and is exposed to actinic lightthrough the mask. After exposure, the plate is treated with a solventthat removes the unexposed material, leaving a resist image whichprotects the resist-covered areas of the metal plate from the acid usedin etching the plate (in the case of letterpress) of from the varioushydrophilic coatings used in lithography.

For this purpose, the image in the mask must be of the highest possiblecontrast, e.g., completely opaque black in the image areas, andcompletely transparent (free from fog) in the unexposed areas. For suchmasks, a film known as "litho" film (or various proprietary namescontaining "lith") is used. Such films are usually silver halideemulsions which produce extremely sharp images of high density andcontrast; they are used in the graphic arts for making line and screenimages, and in some phototype-setting systems. The characteristic curveof a lith film has a steeply rising straight-line portion, and a veryshort toe.

When a litho film is exposed through a halftone screen and developed, itcontains an image comprised of dots. The dots correspond to the areas ofthe film under the transparent areas of the halftone screen and arecomprised of exposed and developed material. In silver halide lithofilms, these dots may be reduced in size by "dot-etching". Dot-etchingis reducing the size of, or "etching" the halftone dots, therebychanging the tone values of the image. In silver halide films, this isdone chemically by treating the films with a silver "solvent". Thismethod is used in lithography when tone values or color strength must bechanged during the photographic steps rather than on the printing plate.Its importance is further illustrated in the processes for correctingcolor work.

When process color work is a part of the daily business of alithographic shop, a certain amount of correction is often necessary toaccurately reproduce the colors in the original. A typical procedurerequires the production of a set of three or four color separationnegatives, and from these negatives, a set of halftone positives ismade. Both of these operations are performed by a color cameraman, andat a later stage, the halftone positives are given to the dot-etchartist for correction.

The dot-etcher chemically treats the halftone positive dots to reducetheir size in selected areas. At some point, the dot-etcher is satisfiedthat the work he has done will produce the necessary color correction inthe reproduction. He then turns the corrected positive over to someoneto make a contact halftone negative from it for platemaking. This stepis often performed by the contact printer. At times, if a deep-etchplate is required, the contact printer will first make a contacthalftone negative, and then, from this negative, make a contact halftonepositive. The reason for this seeming duplication of effort is to get animage with a dot sufficiently optically dense for the platemakingexposure. This type of dot is known as a "hard" dot. The dot-etchingprocess reduces the optical density of silver halide dots and thereforenecessitates the extra step to obtain hard dots.

An improved mask suitable for use as a litho film is needed which willproduce hard dots and which does not involve the expense and red lighthandling requirements of silver halide films. Any such mask, however,needs to have the chaaracteristic of dot-etchability so as to perimttone correction.

Photopolymerizable elements do not normally contain silver which isoxidized in a conventinal dot-etch process. Thin, solvent-processablephotopolymerizable elements containing actinic radiation absorbers andcolorants are known. However, the thickness of such solvent developableelements and amounts of actinic radiation absorbers or colorants thereinare adjusted to transmit sufficient actinic radiation to insure completepolymerization of the layer through to the support thereby insuring goodanchorage of image areas thereto. A new type of photopolymerizableelement had to be invented to achieve a dot-etching capability in theabsence of silver comparable or superior to that achieved with silverhalide-containing films.

In accordance with this invention high contrast, dot-etchablesolvent-processable film elements are provided which comprise a supportbearing a very thin actinic radiation photopolymerizable layer no morethan 0.010 mm thick and having an optical density of at least 3.0 to theactinic radiation, the photopolymerizable layer comprised of anethylenically unsaturated compound capable of forming a high polymer byfree radical initiated, chain propagating, addition polymerization, anorganic polymeric binder, a free radical generating additionpolymerization initiator system activatable by actinic radiation in thespectral region of 300 nm or above, and optionally containing an actinicradiation absorber, the photopolymerizable layer being the outermostlayer of the element or being contiguous to a removable cover sheet oran overcoat layer which is at least partially soluble in or permeeableto a solvent for the photopolymerizable layer.

The photopolymerizable layer will ordinarily have a cover sheet orovercoat layer which is transparent to actinic radiation to protect theelement against oxygen inhibition as well as physical damage. Theinvention therefore provides a high contrast, dot-etchablesolvent-processable lithographic film element comprising in order, (1) asupport, (2) a photopolymerizable layer having a thickness of no morethan 0.0004 inch (0.010 mm) and an optical density of at least 3.0 inthe actinic region, and (3) a removable cover sheet or overcoat layerwhich preferably is transparent to actinic radiation. By the term"overcoat layer" as used throughout is meant a coating in dry form overthe photopolymerizable layer which is at least partially soluble in orpermeable to a solvent for the photopolymerizable layer. A mattingagent, e.g., colloidal silica, corn or rice starch particles, ormixtures thereof, etc., can be present in the overcoat layer.

The elements of the invention are used in a process of imagereproduction having the steps of (1) imagewise exposing to actinicradiation a photopolymerizable layer on a support, the layer containingactinic radiation absorbing material, (2) removing the unexposed areasof the layer (e.g., by washing out from the side of the layer which wasexposed to actinic radiation), leaving an image on the support comprisedof the exposed areas of the layer, and (3) reducing the size of exposedimage areas by removing the edges of the areas on the side of the layerwhich was exposed to actinic radiation.

The photopolymerizable elements of the invention can replace the use ofexpensive silver halide materials for making photographic masks. It hasbeen found that the invention, as defined, provides a photopolymerizableelement with a characteristic curve suitable for lithographicapplications and which produces images which are tone correctable bydot-etching without significant loss of image density. The elements ofthe invention are also characterized by rapid processability,suitability for bright light handling, and production of halftone dotimages that are sharper and easier to reduce in size than those ofsilver halide litho films with greatly reduced operator handling. Theseand other aspects of the improvement in the art provided by theinvention are described hereinafter.

IN THE DRAWINGS

FIG. 1 is a schematic (not to scale) cross-sectional view of an exposedand developed element of the invention with exposed areas, whose edgeshave been undercut by development, remaining on the sheet support.

FIG. 2 is a vertical view of exposed and developed elements showing thepattern of halftone dots composed of exposed, opaque photopolymer. FIG.2A represents 60% dots, whose corners intersect thereby defining holes.FIG. 2B represents 10% dots.

FIG. 3 is a schematic cross-sectional view of an exposed and developedelement of the invention with exposed areas whose undercut edges havebeen removed.

FIG. 4 is a vertical view of exposed and developed elements of FIG. 2with undercut edges removed showing the reduced size of the halftonedots of the exposed photopolymer image.

The present invention is based on the discovery of the process of"etching" exposed areas (especially, halftone dots) ofphotopolymerizable material and that photopolymerizable elements usablein this process can be obtained using a photopolymerizable layer whichis very thin, i.e., not more than 0.0004 inch (0.010 mm) thick, andcontains sufficient actinic radiation absorbing materials such that theoptical density of said layer in (i.e., throughout) the actinic regionis at least 3.0. Imagewise exposure to actinic radiation, e.g., in theregion 300 nm and above, preferably 350 to 400 nm, produces hardenedareas in the photopolymerizable layer corresponding to the transparentareas of the stencil or process transparency through which the elementis exposed. Development of the elements after exposure is preferably bywashout of unexposed areas down to the base with a solvent for theunexposed areas. Continued action of the solvent during developmentundercuts (i.e., removes material beneath the edges of) the exposed,hardened areas; since the high optical density of the layer causes agradient in degree of polymerization or hardening through the thicknessof the layer, with the greatest degree of polymerization or hardening atthe surface of the layer. The remaining image areas consist of ahardened upper skin which rests on a softer undervolume having a lesserdegree of polymerization or hardening. The sheet support of the elementssupports the exposed areas of the developed mask and in combination withthe removable cover sheet or overcoat layer provides protection againstoxygen inhibition on exposure.

Photopolymerizable materials are particularly suitable for the layersince they can be washed out with solvent in unexposed areas andundercut easily. Preferred elements of this invention therefore comprisea transparent support, a transparent cover sheet or overcoat layer, anda single photopolymerizable layer, not more than 0.0004 inch (0.010 mm)thick, between the support and cover sheet or overcoat containing:

a. An ethylenically unsaturated compound capable of forming a highpolymer by free radical initiated, chain propagating, additionpolymerization, 7.5-35% by weight,

b. an organic polymeric binder, 10-75% by weight,

c. a free radical generating addition polymerization initiator systemactivatable by actinic radiation, 0.1-25% by weight, and

d. an actinic radiation absorbing material, 0-40% by weight,

present in such concentration as to impart an optical density to thephotopolymerizable layer of at least 3.0 over at least the spectralrange of 350 to 400 nm.

Particularly preferred are such elements having a thickness of not morethan about 0.0003 inch (˜0.008 mm) having an optical density in thephotopolymerizable layer of at least 4.0.

In a particularly preferred element of this invention, the polymericbinders are selected so that the unexposed photopolymerizablecomposition is soluble in predominantly aqueous solutions, e.g., diluteaqueous alkaline solutions, but upon exposure to actinic radiationbecomes relatively insoluble therein. Typically, polymers which satisfythese criteria are carboxylated, e.g., vinyl addition polymerscontaining free carboxylic acid groups. However, a wide variety ofbinders may be used, as disclosed in prior art photopolymerizablecompositions, where aqueous development is not required.

The elements of the invention are versatile in that they can be used aslitho masks and as "photomasks". A mask suitable for contact speedlithographic work is readily provided by the above elements by, forexample, imagewise exposing the element with a halftone screen throughthe cover sheet or overcoat layer, followed by peeling off the coversheet or dissolving the overcoat layer partially or completely, and thendeveloping by washing out the unexposed areas to leave a suitabledot-etchable mask on the support film. Of course, in this application,dyes or pigments, e.g., colloidal carbon, would be added to thephotopolymerizable layer so that the image would be opaque in both theultraviolet and visible regions of the spectrum.

Alternatively, a suitable "photomask" for use in exposing photoresistsand the like can be provided by a slightly different process. In thisinstance, for example, after removing the cover sheet or completely orpartially removing the overcoat layer, the photopolymerizable layer on atemporary support can be laminated to a dimensionally stable and rigidsupport, for example, optical glass. Then, after lamination andexposure, the temporary support is removed and solvent development ofthe photopolymerizable layer yields the photomask on the rigid support.In this application, the layer may contain dyes or pigments absorbingthroughout at least the 350 to 400 nm range. Thus, the photomask can besubstantially transparent in at least a portion, e.g., a major portion,of the visible region of the spectrum, but be substantially opaque inthe actinic region (i.e., the region of the spectrum used to initiatephotopolymerization). For example, the photopolymerizable layer cantransmit light in the magenta, cyan or yellow regions of the visiblespectrum.

The elements of the invention may be used in the ways described abovedepending on the process of development. Development can be stopped whenholes have been cleaned out down to the base and before there issubstantial undercutting. The elements thus developed may be used as aphotomask for circuit line images, for example, where no etching isdesired. If the coating thickness is too great, undercutting may causeuncontrolled or excessive loss of portions of the image. For use as alitho mask, the element is exposed with a halftone screen and developed.When it is desired to correct tone values in the image, development iscarried out further to unercut the halftone dots in the image and permitsize reduction of the dots. Where development is carried out to theextent causing undercutting, the layer must be very thin so that dots inhighlight areas are not completely removed by the etching process. Ifthe layer is too thick, small dots which comprise a photohardened cappreferably not more than one-third the thickness of the layer resting ona tall column of soluble, unexposed material, are exceedingly fragileand subject to breaking off.

The photopolymerizable layer must have an optical density great enoughto produce with a very thin layer an opaque image useful as a mask.Where dot-etching is to be practiced, high optical density is alsorequired to produce a gradient in degree of polymerization.

In photopolymerizable compositions the molecular weight of at least onecomponent of the composition is increased by exposure to actinicradiation, causing a change in the rheological and thermal properties ofthe exposed areas and rendering the exposed areas relatively lesssoluble in solvents than the unexposed areas, thereby producing asolvent developable image. Photopolymerizable compositions for use inthe invention usually comprise a photoinitiator, an actinic radiationabsorber to opacify the element in the actinic region, a polymerizablemonomer, and a polymeric binder, as described further below.

(a) The Photoinitiator

The photopolymerizable composition contains an organic, free-radicalgenerating system activatable by actinic radiation, i.e., 300 nm andabove, which initiates polymerization of the ethylenically unsaturatedcompound and does not subsequently terminate the reaction. Thefree-radical generating system should have at least one component thathas an active radiation absorption band with a molar extinctioncoefficient of at least about 50 within the range of about 300 to 500nm. "Active radiation absorption band" means a band of radiation whichis active to produce the free radicals necessary to initiate thepolymerization. The free-radical generating system can comprise one ormore compounds which directly furnish free radicals when activated byradiation. It can also comprise a plurality of compounds, one of whichyields free radicals after having been caused to do so by a sensitizerwhich is activated by the radiation.

Photoinitiators which can be utilized in the practice of this inventioninclude aromatic ketones such as benezophenone, Michler's ketone(4,4'-bis(dimthylamino)benzophenone),4,4'-bis(diethylamino)benzophenone,4-methoxy-4'-dimethylaminobenzophenone, 2-ethylanthraquinone,phenathraquinone, and other aromatic ketones; benzoin, benzoin etherssuch as benzoin methyl ether, benzoin ethyl ether and benzoin phenylether, methylbenzoin, ethylbenzoin and other benzoins; and2,4,5-triarylimidazole dimers such as2-(o-chlorophenyl)-4,5-diphenylimidazole dimer,2-(o-chlorophenyl)-4,5-(m-methoxyphenyl)imidazole dimer,2-(o-fluorophenyl)-4,5-diphenylimidazole dimer,2-(o-methoxyphenyl)-4,5-diphenylimidazole dimer,2-(p-methoxyphenyl)-4,5-diphenylimidazole dimer,2,4-di(p-methoxyphenyl)-5-phenylimidazole dimer,2-(2,4-dimethoxyphenyl)-4,5-diphenylimidazole dimer,2-(p-methylmercaptophenyl)-4,5-diphenylimidazole dimer, and the likedisclosed in U.S. Pat. No. 3,479,185, in British Pat. No. 1,047,569,published Nov. 9, 1966, and U.S. Pat. No. 3,784,557.

Particularly useful initiators are the 2,4,5-triarylimidazole dimers(also known as hexaarylbiimidazoles). These are used with a free-radicalproducing electron donor agent, such as 2-mercaptobenzoxazole, leucocrystal violet or tris(4-diethylamino-2-methylphenyl)-methane. Suchsensitizers as Michler's ketone may be added. Various energy transferdyes, such as Rose Bengal and Eosin Y, can also be used. Additionalexamples of suitable initiators are disclosed in U.S. Pat. No.2,760,863. Other useful systems employ a triarylimidazole dimer and afree-radical producing electron donor agent, with or without the use ofa sensitizing compound as described in U.S. Pat. No. 3,479,185. Anotheruseful group of initiators is those mixtures described in U.S. Pat. No.3,427,161.

The free-radical generating system is employed in a concentrationsufficient to initiate polymerization, which is usually about 0.1-25% byweight based on total composition, and preferably about 2 to 20% byweight.

(b) Actinic Radiation Absorber

Compounds, in addition to the photoinitiator system, effective inabsorbing actinic radiation, which for most photopolymerizablecompositions includes at least the range of 350 to 400 nm, can bepresent in the layer in relatively high concentrations to provide a veryopaque mask and cause a polymerization gradient required for imageundercutting. Such a gradient is provided in such compositions by anoptical density of at least 3.0 in the layer over at least the spectralrange of 350 to 400 nm inclusive. Mixtures of actinic radiationabsorbing photoinitiator systems, dyes, and pigments are frequently usedto provide the absorption over the visible and actinic range providingan optical density of at least 3.0 throughout it or at least over therange of 300 to 500 nm.

Examples of ultraviolet dyes, ultraviolet absorbors and other dyes whichcan be used in this invention are listed below:

2,2'-dihydroxy-4-methoxy-benzophenone

4-dodecyloxy-2-hydroxybenzophenone

2,4-dihydroxybenzophenone

hydroxyphenylbenzotriazole

2(2'-hydroxy-5'-methoxyphenyl)benzotriazole

resorcinol-monobenzoate

2-hydroxy-4-methoxybenzophenone

2,2'-dihydroxy-4,4'-dimethoxy-benzophenone

2,2',4,4'-tetrahydroxybenzophenone

2-hydroxy-4-methoxy-benzophenone-5-sulfonic acid

(also sodium salt of above)

ethyl-2-cyano-3,3-diphenylacrylate

2-ethylhexyl-2-cyano-3,3-diphenylacrylate

    ______________________________________                                        Luxol® Orange GRL                                                                       Color Index                                                                              #25 (Solvent Orange)                                 Luxol® Orange GS                                                                        Color Index                                                                              #24 (Solvent Orange)                                 Luxol® Orange R                                                                         Color Index                                                                              #20 (Solvent Orange)                                 Plasto® Orange M                                                                        Color Index                                                                              #21 (Solvent Orange)                                 Plasto® Orange RS                                                                       Color Index                                                                              #22 (Solvent Orange)                                 Grasol®                                                                   Fast Orange 2RN                                                                             Color Index                                                                              #33 (Solvent Orange)                                 Oil Orange    Color Index                                                                              #12055 (Solvent Yellow                                                        #14)                                                 Sudan Orange RA                                                                             Color Index                                                                              #12055 (Solvent Yellow                                                        #14)                                                 Luxol® Yellow G                                                                         Color Index                                                                              #45 (Solvent Yellow)                                 Luxol® Yellow T                                                                         Color Index                                                                              #47 (Solvent Yellow)                                 Plasto® Yellow GR                                                                       Color Index                                                                              #39 (Solvent Yellow)                                 Plasto® Yellow MGS                                                                      Color Index                                                                              #40 (Solvent Yellow)                                 Oil Yellow 3G Color Index                                                                              #29 (Solvent Yellow)                                 Oil Yellow N  Color Index                                                                              #2 (Solvent Yellow)                                  Sudan Yellow  Color Index                                                                              #30 (Solvent Yellow)                                 Luxol® Fast Blue AR                                                                     Color Index                                                                              #37 (Solvent Blue)                                   Luxol® Fast Black L                                                                     Color Index                                                                              #17 (Solvent Balck)                                  Primrose Yellow                                                                             Color Index                                                                              #77603 (Pigment)                                     Chrome Yellow Light                                                                         Color Index                                                                              #77603 (Pigment)                                     Chrome Yellow Medium                                                                        Color Index                                                                              #77600 (Pigment)                                     ______________________________________                                    

Dispersed Manganese dioxide

    ______________________________________                                        Toluidine Yellow GW                                                                          Color Index #71680 (Pigment)                                   Molybdate Orange                                                                             Color Index #77605 (Pigment)                                   Dalamar Yellow Color Index #11741 (Pigment)                                   Green Gold     Color Index #12775 (Pigment)                                   Graphtol Yellow                                                                              Color Index Pigment Yellow #61                                 Graphtol Orange                                                                              Color Index Pigment Orange #13                                 ______________________________________                                    

Colloidal carbon is a particularly preferred pigment.

To obtain an optical density in a thin layer of at least 3.0 throughoutthe 350 to 400 nm region frequently requires a high percentage of dyesand/or pigments, usually between 15-40% by weight, of thephotopolymerizable layer. When the element of this invention is to serveas a mask for only a limited number of exposures to ultravioletradiation a high concentration of photoinitiator alone (0% dyes and/orpigments) is sufficient to provide the required optical density.Preferably, however, to provide a mask suitable for many exposures highconcentrations of ultraviolet stable dyes and/or pigments are present.

(c) The Monomer

The instant invention is not limited to the use of any particularpolymerizable monomer, it being required only that the monomer beethylenically unsaturated and capable of addition polymerization. Alarge number of useful monomers is available, generally characterized byone or more terminal ethylenic groups. Among the suitable monomers maybe mentioned various vinyl and vinylidene monomers, e.g., vinylcarboxylates, alpha-alkyl acrylates, alpha-substituted acrylic acids andesters thereof, vinyl esters, vinyl hydrocarbons, acrylic andalpha-subsituted acrylic acid esters of the polymethylene glycols andether alcohols, all as disclosed in U.S. Pat. Nos. 2,760,863 and2,971,504; the various compounds disclosed in U.S. Pat. No. 2,927,022and especially those having a plurality of addition-polymerizableethylenic linkages, particularly when present as terminal linkages, andmore especially those wherein at least one and preferably most of suchlinkages are conjugated with a doubly bonded carbon, including carbondoubly bonded to carbon or to such hetero-atoms as nitrogen, oxygen andsulfur; esters of pentaaerythritrol compounds of the kind disclosed inU.S. Pat. No. 3,261,686; and compounds of the kind described in U.S.Pat. No. 3,380,831; e.g., the reaction product of trimethylolpropane,ethylene oxide, and acrylic and methacrylic acids.

The concentration of the monomer or monomers employed is normally about7.5-35% by weight based on the total solids of the composition, andpreferably between 15-25%.

(d) The Binder

The binder used is an organic polymeric material that is preferablysolid at 50° C., and it is necessary that the binder be compatible withthe polymerizable monomer and the polymerization initiator system. Itmay frequently be desirable, but it is not required, that the binder bethermoplastic. The binder may be of the same general type as thepolymerizable monomer being used and may be soluble therein andplasticized thereby.

A wide variety of suitable binders, both thermoplastic andnonthermoplastic, is disclosed in U.S. Pat. No. 3,060,023, e.g.,cellulose ethers or esters; polyalkylene ethers; condensation polymersof glycols with dibasic acids; polymers and copolymers of vinyl esters;acrylic acids and esters; polyvinyl alcohol; cellulose; phenolic resins;and the like. Other binders, including a number of vinylidene polymers,are disclosed in U.S. Pat. Nos. 2,760,863 and 2,791,504. Still otheruseful binders are the N-methoxymethyl polyhexamethylene adipamidemixtures of British Pat. No. 826,272, the polyester, polyacetal or mixedpolyesteracetal mixtures of U.S. Pat. No. 2,892,716; the fusiblepolyvinyl alcohol derivatives of U.S. Pat. No. 2,902,365; the fusibleblends of selected organic-soluble, base-soluble cellulose derivativesof U.S. Pat. No. 2,902,365; the fusible blends of selectedorganic-soluble, base-soluble cellulose derivatives of U.S. Pat. No.2,927,022; the polyvinyl acetals having extralinear vinylidene groups ofU.S. Pat. No. 2,902,710; the linear polyamides containing extralinearN-acrylyloxymethyl groups of U.S. Pat. No. 2,972,540; and the1,3-butadienes of U.S. Pat. No. 3,024,180.

The binder or binder mixture usually comprises between 10-75%, byweight, of the photopolymerizable layer.

Particularly preferred as binders are acidic, polymeric, organiccompounds since the photopolymerizable composition resulting isdevelopable in solely aqueous alkaline solvent devoid of organicsolvents. This is advantageous since organic solvents are costly, may behazardous with respect to toxicity and/or flammability, may becomescarce due to petrochemical shortages, and may pollute the air andwater.

One class of film-forming binders which is soluble in aqueous alkalinemedia and is useful in the compositions of the present invention isvinyl addition polymers containing free carboxylic acid groups, whichare prepared from 30 to 94 mole percent of one or more alkyl acrylatesand 70 to 6 mole percent of one or more alpha-beta-ethylenicallyunsaturated carboxylic acids, and more preferably prepared from 61 to 94mole percent of two alkyl acrylates and 39 to 6 mole percent of analpha-beta-ethylenically unsaturated carboxylic acid. Suitable alkylacrylates for use in preparing these polymeric binders include methylacrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methylmethacrylate, ethyl methacrylate, butyl methacrylate, etc. Suitablealpha-beta-ethylenically unsaturated carboxylic acids include acrylicacid, methacrylic acid and the like. Binders of this type, includingtheir preparation, are described in German Application, OS 2,320,849,published Nov. 8, 1973.

The advantages of using acidic binders can also be obtained by selectinga preformed, compatible macromolecular polymeric binding agent which isa copolymer of (1) a styrene-type of a vinyl monomer and (2) anunsaturated carboxyl-containing monomer, as described in detail inBritish Pat. No. 1,361,298.

Another preferred photopolymerizable composition is obtained by using apreformed, compatible macromolecular polymeric binding agent mixture,the components thereof taken from two selected classes. The use of themixtures as described in British Pat. No. 1,507,704, eliminates the needfor organic solvents in developing. These are mixtures of two types ofbinders. The first type is preferably selected from a copolymer of vinylacetate and crotonic acid; a terpolymer of ethyl acrylate, methylmethacrylate, and acrylic acid; and cellulose acetate succinate. Thesecond type is preferably selected from toluene sulfonamideformaldehyde; a copolymer of methyl methacrylate and methacrylic acid; aterpolymer of methyl methacrylate, ethyl acrylate, and hydrogen maleate;a terpolymer of vinyl chloride, vinyl acetate, and maleic acid; acopolymer of styrene and maleic anhydride; and a terpolymer of methylmethacrylate, ethyl acrylate, and methacrylic acid.

Photopolymerizable elements can be prepared by coating thephotopolymerizable compositions from solvents onto supports inaccordance with usual coating practices. Suitable support materialsinclude films composed of high polymers, which are cast as films frommolten polymer, such as polyamides, e.g., polyhexamethylene sebacamide,polyhexamethylene adipamide; polyolefins, e.g., polypropylene;polyesters, e.g., polyethylene terephthalate, polyethyleneterephthalate/isophthalate; vinyl polymers, e.g., vinyl acetals,vinylidene chloride/vinyl chloride copolymers, polystyrene,polyacrylonitrile; and cellulosics, e.g., cellulose acetate, celluloseacetate/butyrate, cellophane. A particularly preferred support materialis polyethylene terephthalate film of the kind described in Alles etal., U.S. Pat. No. 2,627,088, and Alles, U.S. Pat. No. 2,779,684, withor without the surface coating described in the former patent. Thesupport may have a resin "sub" or other layer thereon which may or maynot be soluble and which for purposes of this invention is consideredpart of the support. However, the total thickness of thephotopolymerizable layer and any soluble sub or underlayer should notexceed 0.0006 inch (0.015 mm). By "soluble" is meant solubility in asolvent in which the photopolymerizable layer is developable. Preferredsoluble sub layers have a thickness not exceeding about 0.0002 inch(0.005 mm). Where the particular application does not require that thebase support be transparent, the photopolymerizable composition mayusefully be coated on an opaque support, such as paper, espciallywater-proof photographic paper; thin metal sheets, especially aluminimand copper sheets, cardboard and the like. The optimum coating thicknessfor a particular purpose will depend on such factors as the use to whichthe coating will be put, the particular light-sensitive compositionemployed, and the nature of other compounds which may be present in thecoating. Preferred dry coating thicknesses for the photopolymerizablelayer can be from about 0.0001 to about 0.0003 inch (0.0025 to ˜0.008mm).

Even after evaporation of the solvent, many of the photopolymerizablecoatings made from the various components outlined above are somewhatsoft, sticky, or tacky. To facilitate storage and handling and toprotect against oxygen inhibition during exposure, there is applied aremovable cover sheet or overcoat layer, which may be either apreviously cast film or an additional coating. The cover sheet is anonphotosensitive sheet that it is completely removable from the entirephotopolymerizable layer in one operation, i.e., by stripping. Aconvenient and suitable material for a cover sheet that can bemechanically stripped off is any of the several commercially availablevarieties of polyethylene terephthalate or polypropylene film.Alternatively, polymeric materials, e.g., polyvinyl alcohol, gelatin,etc., may be coated in solution over the photopolymerization stratum toleave, after removal of solvent, a dry overcoat layer, which dependingon its composition, is at least partially soluble in or permeable to thesolvent for the photopolymerizable layer. Some overcoat layers aredescribed in U.S. Pat. No. 3,458,311.

In practicing an embodiment of the invention, an element containing animage-yielding photopolymerizable stratum is made by coating a layer ofa photopolymerizable composition disclosed herein on a suitable filmsupport. After drying the photopolymerizable stratum, there is laminatedto the surface thereof a removable cover sheet or coated thereon anovercoat layer. The photopolymerizable composition is preferably coatedto give a dry coating thickness of about 0.0002 inch (50 mg/dm²) (0.0051mm). A suitable support film may be chosen from a wide variety of filmscomposed of high polymers, e.g., polyamides, polyolefins, polyesters,vinyl polymers, and cellulose esters and may have a thickness of from0.00025 inch (0.006 mm) to 0.008 inch (0.203 mm) or more. If exposure isto be through the support and before removing the support film, it must,of course, transmit a substantial fraction of the actinic radiationincident upon it. If the support film is removed prior to exposure, nosuch restrictions apply. If the support is to remain on the layer, as inthe case of a litho mask, it must be transparent. A particularlysuitable film is a transparent polyethylene terephthalate film having athickness of about 0.004 inch (0.102 mm). Suitable removable coversheets may be chosen from the same group of high polymer films describedabove and can have the same wide range of thicknesses. Where exposure isto be through the cover sheet or layer, the same should be transparentto actinic radiation. A cover sheet of 0.0005 inch thick (0.013 mm)polyethylene terephthalate is especially suitable. Support and coverfilms as described above provide good protection to thephotopolymerizable layer. The overcoat layers have also been describedabove. The support and/or the cover film may have other layers thereonas desired, e.g., a silver halide emulsion layer.

Preferred photopolymerizable layers have a thickness of less than 0.0004inch, e.g., about 0.0003 inch (˜0.008 mm). Thicknesses of 0.0002 inch(0.0051 mm) to about 0.0003 inch (18 0.008 mm) are particularlypreferred. The optical density of the layer, which is the result of thelight absorbance of the radiation absorber, the photoinitiating system,and all other materials in the layer, is at least 3.0 and preferably atleast 4.0. A layer having an optical density of 3.0 in the actinicregion absorbs 99.9% of the incident actinic radiation, while a layerwith an optical density of 4.0 absorbs 99.99%.

In carrying out the process of this invention, a photopolymerizableelement is prepared as already described. The element is then exposed toactinic radiation, preferably through the transparent cover sheet orovercoat layer. The exposure may be by means of a light source which isrich in ultraviolet radiation through a halftone image transparency,e.g., process negative or positive (an image-bearing transparencyconsisting solely of substantially opaque and substantially transparentareas where the opaque areas are substantially of the same opticaldensity). The transparency may also have a line image such asengineering drawings. The image or transparency may or may not be incontact with the surface of the element, i.e., contact exposure orprojection exposure. For exposures through transparent film supports thetime required will range from a few seconds to several minutes dependingon the intensity of the exposing radiation and the inherent photographicspeed of the composition. After exposure, the cover sheet or sheetsupport through which the element was exposed is removed, and theexposed layer with its exposed, hardened areas and its unexposed orunderexposed, unhardened areas is developed by removal of the latterfrom the element. The removal of the unexposed areas may be accomplishedby treating the side of the element which was exposed with the developersolution, thereby washing out the unexposed areas. Alternatively, if anovercoat layer is present instead of a cover sheet, the developer forthe unexposed photopolymerizable layer will remove the overcoat layercompletely above the unexposed areas and at least partially above theexposed areas.

Since free-radical generating initiators activatable by actinicradiation generally exhibit their maximum sensitivity in the ultravioletrange, the radiation source should usually furnish an effective amountof this radiation. Both point and broad radiation sources are effective.Such sources include carbon arcs, xenon arcs, mercury vapor arcs,fluorescent lamps with ultraviolet radiation emitting phosphors, argonglow lamps, electronic flash units and photographic flood lamps. Ofthese, the tungsten 1000-watt quartz iodide lamp is most suitable. Thesunlamp mercury-vapor arcs are customarily used at a distance of 3.8-61cm from the photopolymerizable layer. The point sources are generallyused at a distance of 50-125 cm from the element. It is noted, however,that in certain circumstances it may be advantageous to expose withvisible light, using a photoinitiator sensitive in the visible region ofthe spectrum. In such cases, the radiation source should furnish aneffective amount of visible radiation. Many of the radiation sourceslisted above furnish the required amount of visible light.

After exposure, the element is developed, e.g., by washing out with asuitable solvent the unexposed, unhardened, soluble portions of thephotopolymerizable layer on the side of the layer that was exposed. Thismay be carried out by impingement of spray jets, with agitatedimmersion, brushing or scrubbing, to leave the desired colored,hardened, insoluble image. The result is a resist image which hassufficient opacity in the actinic region to be used as an intermediateelement, "master", or mask for further exposing photosensitivecompositions. The resist image may be used as a mask directly, or it maybe subjected to "etching", which is the chemical undercutting andsubsequent removal of the edges of the image areas. When the image areasare in the form of dots, this process is referred to as "dot-etching".

The developer solvent is a fluid in which the unexposed areas aresufficiently soluble to be removed from the support and in which theexposed areas are relatively insoluble. The developer solvent selectedtherefore depends upon the photopolymerizable composition. Thus, thesolvent used in coating the composition can be used. Weakly acidicphotopolymerizable compositions may be conveniently developed withsolvent mixtures as disclosed in U.S. Pat. No. 3,475,171. The morepreferred aqueously developable photopolymerizable compositions aredeveloped with aqueous bases, i.e., aqueous solutions of water-solublebases in concentrations generally in the range from 0.01% to 10% byweight.

Suitable bases for the development include the alkali metal hydroxides,e.g., lithium, sodium and potassium hydroxide; the base-reacting alkalimetal salts of weak acids, e.g., lithium, sodium, and potassiumcarbonates and bicarbonates; ammonium hydroxide and tetrasubstitutedammonium hydroxides, e.g., tetramethyl-, tetraethyl-, trimethylbenzyl-,and trimethylphenyl-ammonium hydroxides, sulfonium hydroxides, e.g.,trimethyl-, diethylmethyl-, dimethylbenzyl-sulfonium hydroxides, and thebasic soluble salts thereof, e.g., the carbonates, bicarbonates andsulfides; alkali metal phosphates and pyrophosphates, e.g., sodium andpotassium triphosphates and sodium and potassium pyrophosphates;tetra-substituted (preferably wholly alkyl) phosphonium, arsonium, andstibonium hydroxide, e.g., tetramethylphosphonium hydroxide.

The same solvents would also be useful in the dot-etch process, whereinthe partially hardened dot is undercut and then reduced in size bymechanical abrasion.

The elements of the invention provide a cost saving alternative tosilver halide masks. A black, machine processable, contact-speed,photopolymer litho film has been prepared according to the inventionwith properties equivalent or superior to competitive silver halidefilms. For example, a 30-second, contact, tungsten lamp exposure givesimage reproduction equal to that of a silver halide litho film. Theabove exposure is sufficient to reproduce a 150 line/inch (59.05line/cm), 2-98% dot tonal range. Transmission density of backgroundareas is comparable to silver halide (0.05 using blue filter) and gooddeveloper latitude, dot-sharpening, and dot-etching have beendemonstrated for the preferred element; this element comprises a 0.0005inch (0.0125 mm) thich cover sheet of polyethylene terephthalate, a0.0003 inch (0.0076 mm) thick photopolymerizable layer with an opticaldensity of 4.0 in 350 to 400 nm region, containing colloidal carbon,coated on a 0.004 inch (0.102 mm) thick polyethylene terephthalate filmsubstrate, which contains a 0.00002 inch (0.0005 mm) thick solublesubbing. Further, this element can be developed in a low cost, automaticprocessor which consists of an immersion development compartment and awater spray wash section. At the end of the process the film is dried.The whole process requires less than one minute. Finally, soft dotreproduction with the photopolymer film is either equal to or betterthan that of silver halide.

A related product may be prepared which is particularly useful in visualregistration. This product is a colored, visible light-transparent,ultraviolet-opaque, photopolymerizable element which does not require adark room, can accurately reproduce 2-98% dots, 150 lines/inch screen(59.05 line/cm), is dot-etchable, and has good exposure and developmentlatitude.

The photopolymerizable elements of the invention completely displace theuse of silver in litho films thereby providing great cost and handlingadvantages. In addition, the elements have improved resistance toscratching and abrasive markings which ruin masks of the silver type.Advantageously, dyes and colorants added to the layer can be controlledto attain just the optical density needed for further copy preparation.When it is necessary for the technician to use more than one mask tomake a copy, the transparent image type is clearly visible undersafelights used during exposure, resulting in perfect registry. Animportant commercial advantage is that the use of applicants' elementsand processes requires less handling of the photopolymerizable elementsin making masks and that they are dot-etchable.

The etching process may be carried out by continuing development untilsubstantial undercutting of exposed areas has occurred and then reducingthe size of the exposed areas by removing material from the edges oftheir upper surfaces (i.e., the surfaces that were exposed to actinicradiation).

In making masks for exposing litho plates, an exposed and developed maskis usually made and then examined by a color proofer who determineswhether tone correction is required. This can be done by actually makinga litho plate (a "press proof"), by measuring the halftone dots with adensitometer and comparing the colors in the original to color chipskeyed to dot size, by a special color proofing film (as described inU.S. Pat. No. 3,649,268), or by any other suitable method of colorproofing. If it is determined that tone correction is desired, the maskcan be dot-etched by returning it to the developer apparatus which isequipped with spray nozzles or other means to provide the mechanicalaction for dot-etching. The amount of dot-etching will be proportionalto the amount of time in the developer apparatus, so that the process oftone correction by dot-etching can be quite easily controlled. Insteadof returning the mask used for proofing to the developer, a new mask canbe prepared and simply developed for a longer period of time, so as toyield a dot-etched mask of proper tonal balance. With eitheralternative, the process usually involves a size reduction in the dotsof at least 5% between the mask used for proofing and the subsequentlydot-etched mask. More commonly, a size reduction of 10% or more iscarried out with the dot-etch process. As illustrated in the examples, adot-size reduction of 75% or more may be achieved with elements of theinvention.

It it is determined that only a portion of the image on the mask needstonal correction, developer solvent may be applied to that area alone,followed by rubbing with a wet pad or spraying, to dot-etch that areaselectively.

When an element of the invention is exposed with a halftone screen andthen developed, the image thereon is comprised of normally squarehalftone dots of polymeric material. It will be noted that square dotsof a size of 50% or greater touch at the corners and thereby defineclear "holes" in the mask. Dot-etching of such large dots therefore maybe viewed as hole enlargement; however, hole enlargement occurs throughreduction in size of the exposed, polymeric dots. In all casesdot-etching results in less opaque polymeric exposed area on the maskand therefore a reduction in the overall density of the image due tolarger spaces between the opaque polymeric exposed areas. The drawingsillustrate the effect of dot-etching on large and small size dots. FIG.1 represents a dot image that has been undercut but not subjected tophysical abrasion to reduce dot size. FIG. 2A shows 60% dots beforedot-etching, and 2B shows 10% dots before dot-etching. FIG. 3 representsa dot image that has been dot-etched. The upper surfaces of the dotshave been broken off, thereby reducing dot size. FIG. 4A shows the dotimage of FIG. 2A after dot-etching. The dots have been reduced in sizeby physical abrasion, thereby enlarging the holes between them. FIG. 4Bshows the dots of FIG. 2B after dot-etching which has reduced theirsize.

The invention accordingly provides a process comprising the steps ofimagewise exposing the element, removing the unexposed areas of thephotopolymerizable layer, and reducing the size of exposed image areasby removing the edges of said areas. The latter two steps can be carriedout sequentially in a single pass through a developer apparatus suitablyequipped with spary, brushing, or other means to apply mechanical actionto the dot surface. Alternatively, these steps may be carried out byhand, as by immersion in developer and rubbing. The first step of theprocess, imagewise exposing, especially through a halftone screen, thusproduces a dot-etchable mask from an element of the invention. The maskcontains a tone correctable image comprised of opaque polymeric dotswhich are reducible in size by mechanical action (e.g., rubbing,brushing, or spraying) on the image bearing surface of the mask. Such amask before and after size reduction by mechanical action is representedby FIG. 1 and 3, respectively.

As explained above, the major utility of dot-etching is in lithographicplate making, wherein a mask of the invention is one of a plurality ofcolor separation masks and reducing the size of the exposed areas (dots)is carried out to an extent whereby a composite color image producedwith the masks (on a lithographic plate or on a proofing film) has thesame tonal balance as the original color image. The elements of theinvention are capable of functioning in this process, thereby providinga new and improved replacement for silver halide litho masks.

The elements of this invention are also useful as color-codedphotomasks. Such masks can be used to expose photosensitive printingplates which will print yellow, magenta, cyan process colors, andusually also black, thus forming a full-color reproduction of theoriginal. The masks themselves are colored in these shades and thusprovide a simple, rapid method for proofing.

Color-coded photomasks meet the optical density and thicknessrequirements set forth above, i.e., have an optical density throughoutthe actinic region of at least 3.0 and a thickness no greater than 0.010mm. The high optical density is achieved by adding relatively highconcentrations of ultraviolet absorbers to the photopolymerizable layer.Each layer is also colored in a visible region of the spectrum withtransparent dyes or pigments corresponding with inks to be later used;generally yellow, magenta, cyan and black.

In use, a printer desiring to prepare full-color prints of a processtransparency, would expose the yellow photomask of this invention to ahalftone image of the yellow component of the process transparency, themagenta photomask to the magenta component, the cyan photomask to thecyan component, and, usually, a black photomask to the gray-to-blackcomponent. After exposure and development, the imaged masks would beassembled, one on top of the other in register, to yield a full colorproof of the original, e.g., when viewed with transmitted light. If thecolor reproduction was satisfactory, each imaged photomask would be usedfor exposing a positive-working photosensitive printing plate, whichwhen processed and linked with an ink corresponding in color to thecolor of the photomask, would yield prints of that color. Multipleprinting from plates exposed through each of the yellow, magenta, cyan,and black imaged masks, and inked accordingly, would yield a faithfullyreproduced full-color print of the original process transparency.

At other times, when the color-coded imaged photomasks havr beenassembled for proofing, it may be observed that the full color proof is,for example, too yellow, or too red, etc., either overall or in certainregions. In this instance, the imaged photomask corresponding to theexcess color would be corrected by dot-etching, either overall orlocally. After dot-etching, the color-coded imaged photomasks would bereassembled and inspected again. Further corrections may be necessary,and they would be made until the assembled color-coded masks gave thefull-color reproduction desired. When satisfactory, each mask would thenbe used to expose a photosensitive printing plate.

The color-coded photomasks of this invention, when assembled inregister, provide a very convenient proofing system. The abovedescription applies only to positive-working photosensitive printingplates. When negative-working plates are to be exposed, an extra processstep is involved. Each color-coded mask is used to prepare a contactlithographic negative film, which is then used for modulating exposureof the printing plate.

The following examples illustrate embodiments of the invention in whicha photopolymerizable composition, comprising a polymerizable monomer anda polymeric binder is used for the photopolymerizable layer. Suchcompositions have been found to work particularly well in the invention,possibly due to migration of the monomer through the binder toward theradiation source, resulting in a high gradient of polymerization throughthe thickness of the layer and thereby promotion of dot-etchability.Such compositions are therefore particularly preferred.

The developed images of the examples of the invention had high densityin the image areas and little or no density in the nonimage areas. Thelack of density in the nonimage areas (background) provides an importantadvantage over diazotype masks, which have substantial backgrounddensity. This advantage is facilitated by the use of a soluble sublayer. Elements of the invention can be provided which are capable ofhaving an optical density to radiation in the 350 to 400 nm region inthe nonimage areas after development not exceeding 0.3 and preferablybeing virtually nil. Such elements containing a dispersed pigment, e.g.,carbon black, as the radiation absorber are particularly preferred sincethey can provide elements with very good photospeed.

In carrying out the process of this invention, exposures sufficient topolymerize the surface, but not more than about one-third thephotopolymerizable layer, are preferred with the time of exposure notexceeding about 3 minutes.

The invention will be further illustrated by the following examples,wherein parts and percentages are by weight unless otherwise noted.

EXAMPLE 1

A coating composition was prepared from solvent solutions of thefollowing ingredients:

    ______________________________________                                                             Amount    %                                              Component            (grams)   Solids*                                        ______________________________________                                        (a)   Methylene chloride 433.7     0                                                solvent                                                                 (b)   Methanol solvent   47.3      0                                          (c)   Copolymer of methyl                                                                              49.7      40.17                                            methacrylate (90 mole                                                         %) and methacrylic                                                            acid (10 mole %), mol.                                                        wt. range about 30,000-                                                       50,000                                                                  (d)   Polymethylmethacrylate,                                                                          2.6       2.11                                             high mol. wt., 0.9                                                            inherent viscosity                                                      (e)   2,2'-Bis(2-chloro- 8.2       6.60                                             phenyl)-4-4',5,5'-                                                            tetraphenyl biimidazole                                                 (f)   4,4'-Bis(dimethylamino)-                                                                         3.2       2.58                                             benzophenone (Michler's                                                       ketone)                                                                 (g)   Mixed ester of tri-                                                                              1.8       1.42                                             ethylene glycol-                                                              dicaproate and di-                                                            acrylate, refractive                                                          index 1.4460 at 25° C.                                           (h)   Trimethylol propane                                                                              33.3      26.97                                            triacrylate                                                             (i)   2-Mercaptobenzothiazole                                                                          1.2       0.96                                       (j)   Grasol® Fast Orange 2RN,                                                                     23.7      19.19                                            C.I. Solvent Orange 33                                                        (a mixture of Solvent                                                         Red 30, C.I. 27291 and                                                        dicyclohexylamine salts                                                       of a bisazodisulfo acid                                                       dye).                                                                   ______________________________________                                         *in the coating composition                                              

After thorough mixing, a portion of the photopolymerizable compositionthus formed was coated on the sub layer of a 0.007 inch thick (0.178 mm)polyethylene terephthalate clear film support, prepared as described inExample IV of assignee's Alles U.S. Pat. No. 2,779,684, containing aninsoluble resin sub layer on one side only. After hot air drying, thedry photopolymerizable layer was about 0.0003 inch (0.0076 mm) thick andhad an optical density of 4 over the spectral range 300 to 500 nm, asmeasured on a Cary recording spectrophotometer. The resulting productwas laminated with a 0.0005 inch thick (0.0127 mm) clear, biaxiallyoriented and heat-set polyethylene terephthalate film removable coversheet to yield a composite element. This element was then imagewiseexposed for 15 seconds, through the cover sheet side, by a fluorescent,black light source rich in ultraviolet radiation, in a standard (nuArc®)vacuum frame with a silver image transparency bearing an opaque imagesimulating a printed circuit. The radiation source contained 9closely-packed, 24-watt, high output bulbs (General Electric Corp., F24T12-BL-HO); this source was placed 3 inches (7.62 cm) from thephotopolymerizable element. After exposure the cover sheet was strippedoff and the layer developed, by washing out the unexposed, unpolymerizedportions of the coating, using a predominantly aqueous solution of asolvent for the unexposed material of the layer of the followingcomposition:

    ______________________________________                                                               Grams                                                  ______________________________________                                        Sodium carbonate monohydrate                                                                           35.0                                                 Diethylene glycol monobutyl ether                                                                      226.5                                                Distilled water          3290.0                                               ______________________________________                                    

After developing with the above solution, rinsing with water, and airdrying, the resulting film contained an orange, polymeric,ultraviolet-absorbing image which was transparent to visible light buthad an optical density of 4 or over the spectral range 300 to 500 nm.This imaged element, an exact replica of the silver image transparencyoriginal, was useful as a target, stencil, or mask, for preparingprinted circuits using the process and materials described in U.S. Pat.No. 3,469,982.

Another composite element made as previously described was given a 30second exposure to the compact black light source above through a 150line halftone screen hard dot target through the cover sheet side. Afterremoving the cover sheet, the unexposed areas were washed away byimmersing the element in the developer described above at 73°-74° F.(23° C.) for 90 seconds, immersing the element in water to stopdevelopment and then rubbing the element with a developer-laden pad todot-etch and eliminate residual unexposed material. Transmission densityof background areas is comparable to silver halide films (0.05 using ablue filter). The process produced an image on the film supportcomprised of polymeric dots reduced in size from the size correspondingto the halftone screen values. Alternatively, the element could havebeen developed in an apparatus having scrubbing members.

The use of a weak developer improves developer latitude, i.e., theamount by which the time of development can be varied withoutsignificant effect. Immersion in water quickly stops the action of thedeveloper and minimizes the requirement for a wide developer latitude.After immersion in water the element can be rubbed with a water-wet padto remove material which, although not dissolved, remains loose on theelement.

Another composite element made as previously described was dot-etched bythe following procedure. The etching solution used was similar todeveloper with the addition of a surfactant, octyl phenoxy polyethoxyethanol (Triton® X-100), and a thickener (cellulose gum). The testsample was first immersed in etching solution for 10 seconds and takenout and allowed to stand (on horizontal surface) for a variable amountof time, after which it was immersed in water and then rubbed with adeveloper laden pad. Total etch time vs. dot equivalency is given below:

    ______________________________________                                        ETCH TIME - DOT EQUIVALENCY                                                   (Immersion + Standing Time)                                                   Seconds              (Dot Equivalency)                                        ______________________________________                                         0                   92                                                       20                   84                                                       40                   73                                                       80                   63                                                       120                  54                                                       ______________________________________                                    

Microdensitomer Measurements

Measurements were taken in 2 etched areas of the tonal range. The firstwas a 90% test area reduced to a 60% dot equivalency. This area ischaracterized by mostly solid background with circular clear holesdispersed in a regular pattern. Etching produced larger holes whichremain circular. The other area, which is characteristic of the lowtonal range up to 50% dot equivalency, is comprised of a regular patternof dots. Results of microdensitometer scans are given below:

    ______________________________________                                        Original Dot Equivalency                                                                           90        45                                             Element              hole      dot                                            Etched-Dot Equivalency                                                                             60        10                                             Etch Time (sec.)     100       60                                             % Density loss in element                                                                           6        20                                             ______________________________________                                    

This illustrates that the value of the tones can be drastically alteredwithout seriously affecting top density.

150 (59.05) Line Halftones Reproduction

Dot equivalencies of the developed test sample before etching in the5-93% region of the tonal range are shown below:

    ______________________________________                                        Original Silver     Photopolymer                                              Halide Hard Dot     Reproduction                                              Target % Dot*       % Dot                                                     ______________________________________                                         5                   5                                                        10                   9                                                        23                  21                                                        34                  33                                                        43                  41                                                        54                  55                                                        65                  65                                                        77                  76                                                        83                  84                                                        93                  95                                                        ______________________________________                                         *Since the photopolymer system is negative working, these number represen     the anticipated values to be obtained from this original.                

These data show that the extremes of the tone range (5 to 93%) werereproduced simultaneously.

Exposure Latitude

The following data compare exposure latitude of photopolymer testsamples prepared and developed as above, without etching vs. a silverhalide contact litho mask. A test target was exposed at various exposuretimes and four target areas were measured to determine dot equivalency.

    ______________________________________                                        Exposure Photopolymer Mask                                                    (seconds)                                                                              Dot Size                                                             ______________________________________                                         5       6       29      41              91                                   10       9       34      48              94                                   20       9       35      50         A    96                                   40       9       39      54              97                                   80       9       39      68              98                                   ______________________________________                                        Exposure Silver Halide Mask                                                   (seconds)                                                                              Dot Size                                                             ______________________________________                                         5       (top density too low at this                                                  exposure)                                                            10       7       33      53              93                                   20       7       34      54         B    94                                   40       7       37      59              96                                   80       7       40      63              97                                   160      9       44      72               99.sup.+                            ______________________________________                                    

The most critical area in both cases is on the 50% dot size range wherea 12% variation is noted from 10-40 seconds (A & B), indicatingequivalent exposure latitude for the photopolymer mask and the silverhalide mask. It is therefore demonstrated that the elements of theinvention, which contain no silver halide, have equivalent sensitometryfor contact litho mask applications to silver halide masks of the priorart.

EXAMPLE 2

This example shows various applications of the elements of the inventionas masks. A coating composition identical to that described in thepreceding example was prepared but containing, in addition, 42.5 g ofBasic Blue 7 (C.I. 42595). Using the element preparation procedure,exposure device, and development process detailed in Example 1, aphotographic, silver halide negative test image was used as targetduring exposure. The resultant black, polymeric, positive image, havinga thickness of 0.0003 inch (0.0076 mm) and absorbing with opticaldensity greater than 3.0 in the range 300 to 500 nm, was used as a maskin exposing the following light-sensitive materials:

1. A photopolymer printing plate described in U.S. Pat. No. 3,458,311was exposed through this mask for 45 seconds at 36 inches (91.4 cm)under a carbon arc. The line and halftone pattern was developed with thedeveloper composition disclosed in the above reference and a suitableprinting plate was obtained.

2. A "tacky" photopolymer system as described in U.S. Pat. No. 3,649,268was prepared and coated to a dry thickness of 0.0004 inch (0.01 mm) onresin subbed polyethylene terephthalate and overcoated by laminating a0.0075 inch thick (0.019 mm) polyethylene terephthalate sheet on top.Exposure was carried out with a xenon arc for 15 seconds at 18 inches(45.7 cm). The image, which is formed by the hardening of the tackypolymer in the area which was exposed, was developed after removal ofthe polyethylene terephthalate cover sheet by dusting in theconventional manner with a dry, colored dye which adhered to those tackyportions. This material is described in Heiart, U.S. Pat. No. 3,307,943.The resultant dyed imaged was transferred to coated paper stock bylamination of 110° C. and an excellent positive copy of the photomaskwas thus secured.

3. A moderately slow, ortho-sensitive silver halide film was exposed for10 second with the photopolymer mask using a General Electric 100-watt,20-volt standard incandescent lamp at a distance of 53 inches (135 cm)through a 0.6 neutral density filter and a No. 47 Wratten® filter. Theimage was developed in a standard multiprocess type developer(metol-hydroquinone) followed by fixing in a conventional photographicfixer, washed and dried as known to those skilled in the art. A veryhigh resolution black and white image was obtained.

EXAMPLE 3

A black photopolymerizable layer, similar to Example 1, was preparedcontaining the following ingredients:

    ______________________________________                                        Component (of Example 1)                                                                              Percent                                               ______________________________________                                        Binder (c)              38.20                                                 Photoinitiator (e)      5.92                                                  Photoinitiator (f)      2.34                                                  Plasticizer (g)         1.30                                                  Monomer (h)             24.50                                                 Chain transfer agent (i)                                                                              0.87                                                  Orange dye (j)          13.10                                                 Luxol® Fast Black L 13.8                                                  (C.I. Solvent Black 17)                                                       ______________________________________                                    

The above materials were dissolved in methylene chloride/2-ethoxyethanol(7:3 ratio by volume) to yield a coating solution containing 20% solids.This solution was coated on a polymeric film support at a coating weightof 95-100 mg/dm² to yield a dried, photopolymerizable layer with athickness of 0.0004 inch (0.01 mm) and an optical density greater than3.0.

The 0.004 inch (0.10 mm) thick polyethylene terephthalate film support,in this Example, had a 0.0002 inch (0.005 mm) thick soluble substratumwhich was a modification of that disclosed in Rawlins, U.S. Pat. No.3,443,950. This modification consisted essentially of adding two acidicterpolymers in a weight ratio of approximately 1.2:8.01:1 (Rawlinssub:first polymer:second polymer) wherein the first polymer is formedfrom ethyl acrylate (56%), methyl methacrylate (37%) and acrylic acid(7%), and the second polymer is formed from 66% methyl methacrylate, 29%ethyl acrylate, and 5% methacrylic acid, to the substratum described inthe above cited patent.

After completing the element preparation, by laminating the cover sheetof Example 1 following the process of Example 1, the element was exposedthrough hard and soft dot targets. Exposure was carried out for 30seconds with a 2500-watt, pulsed xenon nuArc® (Model FT26M-2) radiationsource at a distance of 43.2 cm. Then, after removing the cover sheet,the black image was developed. The developer of Example 1 was found toperform well with a 30 second immersion at 80° F. (27° C.). An automaticprocessor is used which has a development and a wash stop stage. Afterbeing submerged in a developer section, the film is transported througha water spray which not only removes developed matrix but dilutesresidual developer and stops its action; the element is then dried.Immersion step takes 30 seconds and second stage takes an equal amountof time. Good hard and soft dot reproduction was obtained. The maskimage could be dot-etched if desired.

EXAMPLE 4

A useful, black, solely aqueous alkaline developable, photopolymerizablecomposition was prepared containing the following ingredients:

    ______________________________________                                                                  Amount                                              Component                 (grams)                                             ______________________________________                                        (a)     Solvent (a) of Example 1                                                                            720.0                                           (b)     Solvent (b) of Example 1                                                                            80.0                                            (c)     Terpolymer formed from 56%                                                                          15.2                                                    ethyl acrylate, 37% methyl                                                    methacrylate and 7% acrylic                                                   acid, mol. wt. ca. 260,000,                                                   acid number 76-85.                                                    (d)     1:1 copolymer of styrene                                                                            25.1                                                    and maleic anhydride,                                                         partially esterified with                                                     isopropyl alcohol, mol. wt.                                                   ca. 1700, acid number ca. 270                                         (e)     Photoinitiator (e) of Example 1                                                                     5.3                                             (f)     Photoinitiator (f) of Example 1                                                                     2.1                                             (g)     Chain transfer agent (i) of Example 1                                                               0.8                                             (h)     Triethylene glycol dimethacrylate                                                                   12.0                                            (i)     Dye (j) of Example 1  20.0                                            (j)     Blue dye of Example 2 (C.I. 42595)                                                                  4.0                                             (k)     Nonionic fluorocarbon surfactant                                                                    0.3                                                     FC-430, Minn. Mining and Mfg.                                                 Corp., 10% solution in CH.sub.2 Cl.sub.2                              ______________________________________                                    

This coating solution was coated by the use of a so-called "doctor"knife, to yield a dry 0.0003 inch (0.0076 mm) thick layer having anoptical density greater than 3.0 in the 300 to 500 nm region, onto thesubstrate and subbing layer described in Example 3. Element preparationwas completed by lamination, at 75° C., as described in Example 1. Thefinished product was given a 1 minute exposure, through a halftonetarget, using an xenon lamp (nuArc® "Flip-Top" Plate Maker ModelFT26M-2). Development (15 seconds) with a 4% aqueous sodium carbonatesolution at 80° F. (27° C.) gave an exact positive copy of the originalwhich was useful for preparing other copies.

EXAMPLE 5

This example illustrates a different initiator system and preparing aphotomask on glass:

    ______________________________________                                                           Amount     %                                               Component          (grams)    Solids                                          ______________________________________                                        Methanol           160.0                                                      Methylene chloride 6928.0                                                     Poly(methyl methacrylate)                                                                        351.6      22.60                                           mol. wt. ca. 30,000                                                           Poly(methyl methacrylate)                                                                        233.3      14.99                                           mol. wt. ca. 100,000                                                          Copolymer of methyl                                                                              58.7       3.77                                            methacrylate and gamma-                                                       methacryloxypropyl-                                                           trimethoxy silane, see                                                        U.S. Pat. No. 3,758,306                                                       Photoinitiator (f) of                                                         Example 1          2.42       0.0156                                          Benzophenone       11.2       0.72                                            Triethylene glycol                                                            diacetate          98.5       6.33                                            Monomer (h) of Example 1                                                                         410.0      26.35                                           Dye (j) of Example 1                                                                             389.0      25.0                                            ______________________________________                                    

This solution contained about 13.7% solids and was doctor knife coatedon 0.001 inch thick (0.025 mm) clear, biaxially oriented and heat setpolyethylene terephthalate film. After drying, the film was laminatedwith 0.001 inch thick (0.025 mm) polyethylene to protect the resultingUV-absorbing layer which had a dried thickness of about 0.0003 inch(0.0076 mm).

This UV-absorbing layer having an optical density greater than 3.0 inthe 300-500 nm region was next transferred to a 3"×4" (7.6×10.2 cm)projector slide cover glass, which had been thoroughly cleaned byscrubbing with solvents. Transfer was accomplished by removing thepolyethylene cover sheet and laminating the UV layer to the glass platethrough heated rollers at 100°-120° C. at approximately 183 cm/min.

The photopolymerizable layer was then given a 10 second exposure at 40.6cm with a 1000-watt quartz iodide lamp through a negative containing astandard test pattern comprised of simulated electronic printedcircuitry. The polyethylene terephthalate sheet was removed bystripping, and the resulting image developed by spraying 10 seconds withmethyl chloroform, followed by a water spray, and the resulting orangeimage dried by blowing with compressed air. The solvent used hereindissolved the unhardened areas of the photoresist, leaving a suitable,positive mask on a rigid glass support useful for imaging photoresistswith light sensitivity of between 300 to 500 nm.

EXAMPLE 6

An orange photopolymerizable composition, similar to Example 4, butcontaining an oxygen impermeable cover layer, was prepared containingthe following ingredients:

    ______________________________________                                                            Amount     %                                              Component of Example 4                                                                            (grams)    Solids                                         ______________________________________                                        Solvent (a)         363.0      0                                              Solvent (b)         41.4       0                                              Binder (c)          15.8       17.96                                          Binder (d)          26.2       29.69                                          Photoinitiator (e)  5.5        6.21                                           Photoinitiator (f)  2.2        2.46                                           Chain transfer agent (g)                                                                          0.83       0.94                                           Monomer (h)         12.6       14.30                                          Dye (i)             25.0       28.36                                          Surfactant (k)      0.7        0.08                                           ______________________________________                                    

The above coating solution was coated on the substrate described inExample 3, to give a dried coating weight of 80-90 mg/dm² with athickness of about 0.0003 inch (0.0076 mm). An oxygen barrier layer,which had previously been coated on the cover sheet described in Example1, was then laminated over the above photosensitive layer. The barriercomposition, dry coating weight 6-10 mg/dm², was a modification of anaqueous-alcoholic solution of polyvinyl alcohol (medium viscosity, 99%saponified) containing 2% of a polyoxyethylene surfactant as disclosedin U.S. Pat. No. 3,458,311, Example 1. The modification comprises addingto the polyvinyl alcohol, in a ration of 5:1, a copolymer ofvinylpyrrolidone and vinyl acetate (89/11); the viscosity of a 15%ethanol solution of this copolymer ranges between 38-44 centistokes at75° F. (24° C.). In addition, to insure good wettability, the barrierlayer contains a small amount of a surfactant of the followingstructure: ##STR1##

After removing the cover sheet, the removable oxygen-barrier layerremains on the photosensitive layer. The element is then exposed,through a process transparency, by the black light device of Example 1for 30 seconds. An orange image of the original suitable for use as aphotomask, was obtained on developing for 80 seconds at 68°-70° F. (20°C.), by immersion in a developer composition containing 2.33% sodiumbicarbonate and 0.67% sodium carbonate in distilled water, followed bywashing with water, scrubbing with a water-laden pad, then drying. Thedeveloper removes the oxygen-barrier layer and then the unexposed areasof the photopolymerizable layer. The following data show that asdevelopment is continued for longer periods of time, etching of the lineimage occurs:

    ______________________________________                                        DEVELOPMENT vs. LINE WIDTH                                                    Immersion Time       Line Width                                               (secs.)              (microns)                                                ______________________________________                                        40                   124 (underdev.)                                          45                   124                                                      50                   119                                                      55                   117                                                      60                   111                                                      ______________________________________                                    

Original line target=124 microns clear space.

EXAMPLE 7

This example demonstrates the necessity of high optical densities inorder to achieve dot-etchability. Four photopolymerizable compositionswere prepared as described in Example 6. Composition A is identical withExample 6; Compositions B-D vary as follows:

    ______________________________________                                                 Composition Variations (grams)                                       Component  A         B         C       D                                      ______________________________________                                        CH.sub.2 Cl.sub.2                                                                        363.0     340.8     318.0   279.3                                  CH.sub.3 OH                                                                              41.0      37.9      35.3    31.0                                   Michler's                                                                     ketone     2.2       1.65      1.1     0.55                                   Dye        25.0      20.0      15.0    6.25                                   Optical    4         3         2       1                                      Density                                                                       300-500 nm                                                                    ______________________________________                                    

After exposure, as in Example 6, to a target containing 125 micronlines, the decrease in line widths with increasing development wasmeasured, using the developer and process of Example 6. The followingdata were obtained.

    ______________________________________                                        Development  Line Widths (μ)                                               Time (sec.)  A        B        C      D                                       ______________________________________                                        40           118      120      117    120                                     45           114      119      118    120                                     50           112      114      112    120                                     55           109      --       117    120                                     60           108      112      --     120                                     ______________________________________                                    

The results indicate that Composition A (Ex. 6), with an O.D. of 4 over300 to 500 nm, is dot (line) etchable in a regular pattern. CompositionB (O.D. of 3), is also dot-etchable, but less readily so. Composition C(O.D. of 2) gave somewhat erratic results, but is obviouslysubstantially not dot-etchable. Composition D (O.D. of 1) is clearlystabilized, presumably polymerized, down to the base, and thus notetchable (undercut) at all.

EXAMPLE 8

Part A

This example illustrates a photopolymerizable, aqueously developable,contact-speed, lithographic system containing a pigment. The followingcoating dispersion, 10% solids in methylene chloride, was prepared:

    ______________________________________                                                                  %                                                   Component                 Solids                                              ______________________________________                                        (a)    Binder (c) of Example 4                                                                              16.4                                            (b)    Binder (d) of Example 4                                                                              30.3                                            (c)    Photoinitiator (e) of  5.8                                                    Example 4                                                              (d)    Photoinitiator (f) of  2.3                                                    Example 4                                                              (e)    Surfactant (k) of      0.09                                                   Example 4                                                              (f)    Monomer (h) of Example 1                                                                             27.2                                            (g)    Plasticizer (g) of Example 1                                                                         1.7                                             (h)    Colloidal carbon       16.4                                            ______________________________________                                    

In preparing the above coating formulation, components (b) through (g)are dissolved directly in the solvent. Components (a) and (h) are mixedseparately in CH₂ Cl₂, sand milled to obtain a small particle size, thenthe other components are added to this dispersion. Many carbon blackscan be used; furnace black, particle size ca. 75 nm was used in thisexample. The well-mixed coating composition was coated on the substrateof Example 3 to yield a layer with a dry thickness of 0.0002 inch (0.005mm) having an optical density of greater than 3.0 in the 300 to 500 nmregion, and laminated with the cover sheet of Example 1. The element wasthen exposed, through a halftone target, by a BRH lamp (1000-watt,tungsten-iodide), for 30 seconds, the lamp was at a distance of 1 meterfrom the photopolymerizable layer. After removal of the cover sheet, ablack image was obtained by developing the film for 45 seconds at roomtemperature with an aqueous developer containing 2% each of Na₂ CO₃ andNaHCO₃, followed by a water rinse and drying. Further, the positivehalftone dots obtained were dot-etched by swabbing the film in the samedeveloper for 30 seconds. Thus, this film provides a suitablelithographic-type element for preparing further copies.

Part B

Elements, similar to that described in Part A, were used to illustratedot-etching as performed in the printing trade. A positive image master(mask) was made with the elements for each of the four standard colors(magenta, cyan, yellow and black) normally used to prepare plates forprinting a color picture. These elements were exposed using the devicein Example 8 and developed in an automatic processor described inExample 3 with the developer solution described in Examples 9-11 toprovide four dot-etchable masks. A full color proof on color proofingfilm (as described in U.S. Pat. No. 3,649,268) was made from these masksto simulate a press print. On analyzing the proof it was observed thatthere was too much yellow in all parts of the picture and that one areaappears "too red". To correct these problems the yellow positive imageshould be "flat etched" (all the dots in the picture reduced a smallpercentage in size) and the magenta positive image should be "locallyetched" (the dots in the too-red areas reduced a substantial percentagein size).

To accomplish this, copies were made of the yellow and magenta mastersusing the same exposure and development techniques (the positive imagemasters were copied to negative imagwe intermediates which were thencopied to positive image duplicates of the originals). The resultingyellow duplicate was then rerun through the processor with twice thenormal development time. This served to dot-etch all the dots on theimage. Examples of the amount of reduction in the size (coverage) ofvarious dots (150 line screen) are:

    ______________________________________                                        Dot size in         Dot size in                                               Orig. Mask          Etched Mask                                               ______________________________________                                         98.3%               92.1%                                                    55.4                35.0                                                      11.0                 2.3                                                      ______________________________________                                    

Measurements of the change in surface area of the dots undermagnification were compared to the change in integrated density for agiven area. These measurements confirmed that the change in integrateddensity was totally a result of the change in dot area and not a loss ofdensity within the dot.

The magenta positive image duplicate was then locally etched. The areato be etched was visually ascertained, and the rest of the image wasprotected by painting with a "staging solution" containing:

    ______________________________________                                        CCl.sub.2 FCClF.sub.2    240 g                                                Poly(n-butylmethacrylate)                                                                               15 g                                                ______________________________________                                    

Inherent viscosity=0.53 in a solution containing 0.25 g polymer in 50ml. CHCl₃ measured at 20° C. using a No. 50 Cannon-Fenske viscometer.

    ______________________________________                                        Tris-(4-diethylamino-o-tolyl)methane                                                                     0.5 g                                              C.I. Solvent Blue 36       0.3 g                                              ______________________________________                                    

When the stage had dried, the designated area was etched by immersingthe whole film in a solution of 65% Butyl Carbitol®, 17.5% ethyleneglycol and 17.5% water, by volume, for 10-20 seconds and then rinsedwith an impact water spray. The dot size was measured and the processwas repeated until the proper size dot was achieved. In this instance,the reduction was from 40% dots to 10% dots and again there was no lossin density within the dot when tested by comparing integrated densityand dot size. The film was dried by hot air and the stage was removed bycarefully wiping with solvent (CCl₂ FCClF₂) and then dried.

Another full color proof was made from the master cyan and black masksand the etched yellow and magenta masks, which confirmed that therequired color shift had been accomplished. Thus, these masks may beemployed for exposing photopolymerizable printing plates which, in turn,would give a full color press print with the correct color balance.

EXAMPLES 9-11

The following examples are similar to Example 8, but illustrate the useof multiple monomers and alternative photoinitators. The followingphotopolymerizable coating compositions, about 15% solids in CH₂ Cl₂,were prepared:

    ______________________________________                                                       Amount                                                                        (grams)                                                        Component        Ex. 9     Ex. 10    Ex. 11                                   ______________________________________                                        (a) Binder (c)       20.0      10.8    10.8                                       of Example 4                                                              (b) Binder (d)       38.7      19.0    19.0                                       of Example 4                                                              (c) Photoinitiator (e)                                                                             11.4      --      --                                         of Example 4                                                              (d) Photoinitiator (f)                                                                             2.8       --      --                                         of Example 4                                                              (e) Surfactant (k)   0.5       1.0     1.0                                        of Example 4                                                              (f) Monomer (h)      2.3       2.0     2.0                                        of Example 1                                                              (g) Colloidal carbon 16.4      7.2     7.2                                    (h) Monomer (h)      22.1      9.8     9.8                                        of Example 4                                                              (i) Phenanthrenequinone                                                                            --        1.5     --                                     (j) Benzoin methyl ether                                                                           --        --      1.5                                    ______________________________________                                    

As in Example 8, the carbon and binder (a) were finely dispersed bymilling, and the other components added to this dispersion. Then, thewell-mixed coating compositions were coated separately on the substrateof Example 3, and laminated with the cover sheet of Example 1. The driedcoating thickness of Example 9 was ca. 0.004 mm, and of Examples 10-11ca. 0.0076 mm. The Example 9 film had a visible optical density of 3.0,and 4.0 in the region 300 to 500 nm; the films of Examples 10-11 hadoptical densities 4.0 in both the visible and UV regions of thespectrum.

The Example 9 film was exposed for 20 seconds as in Example 8, but withthe radiation source 152.5 cm from the sample. The films of Examples10-11 were exposed for 10 minutes to a xenon lamp using the devicedescribed in Example 4. Then, upon removal of the cover sheet, the filmof Example 9 was developed in the automatic processor of Example 3, 5seconds, 27° C., using as developer a solution of NaHCO₃ (4.6 g) and Na₂CO₃.H₂ O (27.4 g) in water (1 liter), followed by a water spray rinseand drying. The resulting film image was an accurate reproduction of thehalftone target; the "hard" dots exhibited sharp edges, and weredot-etchable.

The film of Examples 10-11 were developed in a tray (15 seconds, 24° C.)in an aqueous solution containing Na₂ CO₃ (2%) and NaHCO₃ (1%). After awater rinse and drying, sections of the imaged films were selectivelydot-etched producing a dot size reduction of approximately 5-10% usingan aqueous solution containing Na₂ CO₃ (0.4%) and NaHCO₃ (0.4%).

EXAMPLES 12-13

The following two examples, also similar to Example 8, illustratealternative binders useful in the invention. Coating compositionscontaining the following components, in CH₂ Cl₂ /2-ethoxy-ethanol, wereprepared:

    ______________________________________                                                            Amount (% of Solids)                                      Component             Ex. 12     Ex. 13                                       ______________________________________                                        (a)  Binder (c) of Example 4                                                                            46.7       --                                       (b)  Photoinitiator (e) of                                                                              7.5        7.5                                           Example 4                                                                (c)  Photoinitiator (f) of                                                                              2.1        2.5                                           Example 4                                                                (d)  Surfactant (k) of    0.1        0.1                                           Example 4                                                                (e)  Monomer (h) of Example 4                                                                           20.0       26.0                                     (f)  Colloidal carbon     18.0       17.6                                     (g)  Copolymer of methyl  5.2        --                                            methacrylate (90%) and                                                        methacrylic acid (10%),                                                       mol. wt. ca. 50,000                                                      (h)  High molecular weight                                                                              --         46.5                                          tetrapolymer from methyl                                                      methacrylate (30%),                                                           butyl acrylate (25%),                                                         acrylonitrile (30%) and                                                       methacrylic acid (15%)                                                   ______________________________________                                    

Following the milling, mixing, coating and lamination procedures andusing the exposure device of Example 8, the two films were exposed for15 and 10 seconds, respectively. The film of Example 12 had a thicknessof 0.00025 inch (0.0063 mm) and O.D. >3, and that of Example 13 0.0002inch (0.0051 mm) and O.D. >3. Following development as described inExample 10 (pH=10.3, 29° C., 10-15 seconds), the dried film halftoneimages were dot-etched. The etching solution was identical to thedeveloper, but was diluted with 3 parts of water. The etching procedureinvolved protecting portions of the film with tape, then gently swabbingthe unprotected area with a cotton pad soaked in etching solution (roomtemperature), followed by a water rinse and drying. Of course, thisetching procedure could be repeated until the desired effect isachieved. As the following data indicate, both films are dot-etchablewith no loss in top density (i.e., the density of the dots). The drop inthe overall (visible) density of the elements indicates dot-etching(i.e., reduction in size of the dots of the image).

    ______________________________________                                        Optical Density (O.D. Units)                                                                       Overall Density,                                         Top Density          50% Dots                                                 Ex.    Before     After      Before   After                                   No.    Etching    Etching    Etching  Etching                                 ______________________________________                                        12     3.5        3.5        0.27     0.21                                    13     3.0        3.0        0.30     0.18                                    ______________________________________                                    

EXAMPLES 14-15

The following two examples, yielding useful orange photomasks similar toExample 4, illustrate the use of alternative dyes. The following coatingcompositions were prepared:

    ______________________________________                                                          Amount (grams)                                              Component of Example 4                                                                            Ex. 14    Ex. 15                                          ______________________________________                                        Solvent (a)         81.0      81.0                                            Solvent (b)         9.0       9.0                                             Binder (c)          1.58      1.58                                            Binder (d)          2.62      2.62                                            Photoinitiator (e)  0.55      0.55                                            Photoinitiator (f)  0.22      0.22                                            Chain transfer agent (g)                                                                          0.08      0.08                                            Monomer (h)         1.26      1.26                                            Surfactant (k)      0.07      0.07                                            Dye A*              0.9       0.9                                             Dye B*              0.5       --                                              Dye C*              1.75      1.75                                            Dye D*              --        0.5                                             ______________________________________                                         *Dyes A-D are all solvent soluble and are dicyclohexylamine salts of acid     dyes. Dye A is solvent Red 30, C.I. 27291. Dye C is the salt of               Tartrazine, C.I. 19140. Dye D is the salt of C.I. 19135. Dye B is the sal     of the acid dye formed on diazotizing and coupling                            2amino-5-chloro-benzenesulfonic acid to                                       3methyl-1-(2,5-dichloro-4-sulfophenyl)-pyrazolone.                       

The above coating solutions were coated, laminated and exposed as inExample 4. Dry coating weights were 118-125 mg/dm², ca. 0.0004 inch(0.010 mm) thick; both were exposed 3 minutes. Development, followingremoval of the cover sheet, employed an aqueous solution containing 2%sodium bicarbonate and 0.67% sodium carbonate, for 2 minutes, at 21° C.After rinsing with water and drying, the resulting orange image had anO.D. >4 in the region 300-500 nm.

EXAMPLE 16

Carbon-pigmented photopolymerizable layers are oxygen-sensitive and mustbe protected from oxygen of the air during exposure. This is frequentlyaccomplished by use of a thin removable film, which is laminated to thesurface of the photopolymer layer. It can also be accomplished byovercoating the photopolymer layer with a solution of an oxygen barrierpolymer, which adheres to the photopolymer layer (after evaporation ofits solvent), gives good oxygen protection, and is removable in thedeveloper solvent. An example of such an overcoat solution is givenbelow.

    ______________________________________                                        Water                    1240.0  g                                            Polyvinyl alcohol (98-99%                                                                              200.0   g                                            saponified, low viscosity)                                                    Copolymer of vinyl pyrrolidone/                                                                        24.4    g                                            vinyl acetate (60/40, medium                                                  mol. wt.)                                                                     Ethyl Cellosolve         24.6    g                                            Polyoxyethylene surfactant                                                    of the formula:                                                                ##STR2##                2.3     g                                            Denatured alcohol        17.3    g                                            ______________________________________                                    

For use in coating a composition similar to Example 8, Part A, asolution containing 3.5% solids was prepared as follows:

    ______________________________________                                        Water                  683.0    g                                             Overcoat solution above                                                                              210.0    g                                             Surfactant above       5.4      ml                                            (10% aqueous solution)                                                        Blue Pigment (Inmont Blue 3G)                                                                        1.8      g                                             ______________________________________                                    

This solution was coated on elements similar to that of Example 8, PartA, but without a cover film, using an extrusion die, the coating elementwas dried at 200° F. (93.3° C.). In three coatings, dried coatingweights of 9.2, 15.0 and 24.1 mg/dm² were obtained. These three filmswere then compared to the control, a composition similar to Example 8,Part A (with a protective cover film). The two overcoated elements withlower coating weights exhibited lower photospeed than the control;furthermore, these two were rapidly affected by the atmosphere (theovercoat lost its gloss and cracks appeared). The element with thehighest overcoat weight, however, exhibited good stability and with bothphotospeed and dot range equivalent to the control.

EXAMPLE 17

500 g of polyvinyl alcohol (98-98.8% saponified, now viscosity) wereadded to 5000 g of distilled water and heated at 85° C. for 2 hours. 100g of the solution was mixed with 262 g distilled water, 18 gpolyoxyethylene surfactant (10% aqueous solution) described in Example16, 10 g ethyl Cellosolve, and 10 g ethyl alcohol. To 100 g of the abovemixture was added 2.7 g of a 30% colloidal silica dispersion containingparticle sizes in the range of 12 to 15 millimicrons and 30 g ofdistilled water. This resulted in an overcoat formulation in grams asfollows:

    ______________________________________                                        Polyvinyl alcohol        2.25                                                 Distilled water          122.50                                               Polyoxyethylene surfactant                                                                             0.45                                                 Ethyl cellosolve         2.50                                                 Ethyl alcohol            2.50                                                 Colloidal silica         2.70                                                 ______________________________________                                    

A photopolymer composition as described in Example 9 was used to preparea photopolymerizable element also described in Example 9.

The cover sheet was removed. Using a 2-mil (0.05 mm) doctor knife, theabove overcoat solution was coated directly on the photopolymerizablesurface and allowed to dry. The coating weight of the overcoat was 10.0mg/dm².

The overcoated element was exposed and developed as described in Example9. It was found that the overcoated element and an element having acover sheet as in Example 9 had substantially the same photospeed(within one ∛2 step).

The following tests were conducted to determine retention of theovercoat:

A. GRAVIMETRIC

A sample of the overcoated film was given an overall imaging exposure(10 seconds). It was found that only 9.0 mg/dm² of overcoat could bedissolved off this sample (as opposed to 10.0 gm/dm² for unexposedfilm). This indicates that 10% of the overcoat remains in the exposedareas.

B. X-RAY FLUORESCENCE

The above gravimetric data are in good agreement with X-ray fluorescenceanalysis, which indicates that exposed and processed image areas retain12% of the silica initially present in the raw-stock.

C. SCANNING ELECTRON MICROSCOPY (SEM)

Additional evidence for overcoat material remaining on exposed andprocessed film is given by scanning electron microscopy. SEM studies offilms overcoated with polyvinyl alcohol based overcoats that contained1-4% of 4 micron silica particles show that significant numbers of theseparticles remain on exposed and processed image surfaces.

EXAMPLE 18

Into a 3-gallon (11.36 liter) emulsion can is placed 4500 g of distilledwater and 100 g of decationized gelatin, and the mixture is allowed tosoak for 15 minutes at room temperature followed by heating to 125° to130° F. (52° to 55° C.) for 30 minutes. The mixture is cooled to 95° to100° F. (35° to 39° C.). The following components are added:

    ______________________________________                                        Component                Amount                                               ______________________________________                                        Ethanol                  50      g                                            Ethyl Cellosolve         50      g                                            10% Aqueous solution of the                                                   surfactant described in Example 16                                                                     50      ml                                           Methyl methacrylate (66%)/ethyl                                               acrylate (29%); acrylic acid (5%)                                             (30% solids in water),   75.0    g                                            Mucochloric acid, 2% aqueous solution                                                                  600.0   ml                                           ______________________________________                                    

The above solution is coated on the surface of photopolymerizable layerof a photopolymerizable element described in Example 9 to a dry coatingweight of 10 mg/dm². The gelatin layer is dried at 230° F. (110° C.).

Two samples of the coating, one square decimeter, are exposed, front andback sides for 1 minute each as described in Example 9, and the weightsof the samples are recorded with the following results:

    ______________________________________                                                             Amount                                                   Sample No.           (grams)                                                  ______________________________________                                        1                    1.4347                                                   2                    1.4393                                                   ______________________________________                                    

Sample 1 and 2 are processed as described in Example 9. After drying,the two samples are reweighed with the following results.

    ______________________________________                                        Sample No.       1           2                                                ______________________________________                                        Initial weight   1.4347      1.4393                                           Weight after     1.4339      1.4386                                           processing                                                                    Weight loss      0.0008      0.0007                                           ______________________________________                                    

The average weight loss is 0.75 mg/dm². This example illustrates thatapproximately 92.5% by weight hardened gelatin remains on thephotopolymerized surface of a photopolymerizable element. The unexposedunpolymerized photopolymerizable layer and its gelatin overcoat arecompletely removed during processing.

EXAMPLE 19

This example illustrates preparing color-coded photomasks of thisinvention useful both as an alternative proofing system and forpreparing printing plates for full color printing.

Three coating solutions were prepared, each containing the followingingredients:

    ______________________________________                                                                 Amount                                               Component of Ex. 4       (grams)                                              ______________________________________                                        Solvent (a)              81.0                                                 Solvent (b)              4.0                                                  Photoinitiator (e)       1.0                                                  Photoinitiator (f)       0.1                                                  Binder (c)               4.1                                                  Binder (d)               6.6                                                  Monomer (h)              3.2                                                  Surfactant (k)           0.04                                                 Ultraviolet absorber     1.0                                                  (2,2'-dihydroxy-4-methoxybenzo-                                               phenone)                                                                      ______________________________________                                    

To each individual solution was also added one of the following dyes:

    ______________________________________                                                                Amount                                                Dye                     (grams)                                               ______________________________________                                        Grasol® Fast Brill. Red RL                                                                        1.0                                                   (C.I. Solvent Red 86)                                                         Irgacet® Yellow 2RL 1.0                                                   (C.I. Solvent Yellow 91)                                                      Ingacet®  Brill. Blue 2GLN                                                                        1.0                                                   (C.I. Solvent Blue 48)                                                        ______________________________________                                    

Each of the above three solutions was then coated, via a "doctor" knife,onto the substrate and subbing layer described in Example 3. After hotair drying, the dried coating weights were approximately 150 mg/dm², thedried coating thicknesses approximately 0.0004 inch (0.01 mm). Eachphotopolymerizable layer was then overcoated with a thin oxygen barrierpolymer, substantially as described in Example 17. The resulting threeelements of the invention all absorbed strongly throughout the 300 to400 nm spectral range; the optical densities varied from 3.0-3.2. Themaximum absorption in the visible portion of the spectrum occurred at675 and 630 nm for the cyan film, at 560 and 450 nm for the magentafilm, and at 470 nm for the yellow film.

Each colored element was then exposed as described in Example 4, butwith a 90-second exposure to the halftone color separation negativecorresponding to its color, i.e., magenta, yellow and cyan. Development(15 seconds) with an aqueous solution (pH 10.4) of potassium carbonate(1.5%) and potassium bicarbonate (1.5%) at 29° C., followed by rinsingin warm water (38° C.), and drying, gave a faithful image of thehalftone targets, one in magenta, one in yellow, and one in cyan.

The cyan film was dot-etched by soaking the imaged film in the developerdescribed above (30 seconds, 29° C.) followed by a spray with warm waterat 38° C. After drying, measurement showed a dot size reduction of a 50%dot to a 45% dot, without any loss of density within a dot. In a similarmanner, the yellow and magenta films can be dot-etched.

We claim:
 1. A high contrast, dot-etchable solvent-processablelithographic film element comprising a support bearing a very thinactinic radiation photopolymerizable layer no more than 0.010 mm thickand having an optical density of at least 3.0 to the actinic radiation,the photopolymerizable layer comprised of an ethylenically unsaturatedcompound capable of forming a high polymer by free radical initiated,chain propagating, addition polymerization, an organic polymeric binder,a free radical generating addition polymerization initiator systemactivatable by actinic radiation in the spectral region of 300 nm orabove, and optionally containing an actinic radiation absorber, thephotopolymerizable layer being the outermost layer of the element orbeing contiguous to a removable cover sheet or an overcoat layer whichis at least partially soluble in or permeable to a developer solvent forthe photopolymerizable layer.
 2. An element according to claim 1 whereinthe layer has an optical density of at least 3.0 in the 300 to 500 nmregion.
 3. An element according to claim 1 wherein thephotopolymerizable layer is no more than 0.008 mm thick.
 4. An elementaccording to claim 1 wherein the layer has an optical density of atleast 4.0 in the 350 to 400 nm region.
 5. An element according to claim1 wherein the photopolymerizable layer is contiguous to a removablecover sheet or overcoat layer.
 6. An element according to claim 1wherein the photopolymerizable layer is the sole photopolymerizablelayer in the element.
 7. An element according to claim 1 wherein thesupport comprises a sheet bearing a nonphotosensitive sub layer betweenthe sheet and the photopolymerizable layer, the sub layer being solublein a developer solvent for the photopolymerizable layer, the combinedthickness of the sub layer and photopolymerizable layer being no morethan 0.015 mm.
 8. An element according to claim 1 wherein thephotopolymerizable layer contains a colorant taken from the group ofblack, yellow, magenta and cyan.
 9. An element according to claim 1wherein the support is a polymeric film.
 10. An element according toclaim 6 wherein the support comprises a sheet bearing anonphotosensitive sub layer between the sheet and the photopolymerizablelayer, the sub layer being soluble in a developer solvent for thephotopolymerizable layer, the combined thickness of the sub layer andphotopolymerizable layer being no more than 0.015 mm.
 11. An elementaccording to claim 10 wherein the photopolymerizable layer has anoptical density of at least 4.0 throughout the 350 to 400 nm region. 12.An element according to claim 1 wherein the photopolymerizable layercomprises an organic free-radical generating addition polymerizationinitiator system activatable by actinic radiation present in an amountof 0.1 to 25%, an actinic radiation absorber present in an amount of 0to 40%, an ethylenically unsaturated compound present in an amount of7.5 to 35%, and an organic polymeric binder present in an amount of 10to 75%, the percentages being by weight of the photopolymerizable layer.13. An element according to claim 1 wherein the photopolymerizable layeris developable in predominantly aqueous developer solvents.
 14. Anelement according to claim 5 wherein the overcoat layer is transparentto actinic radiation.
 15. An element according to claim 9 wherein thepolymeric film support is transparent to actinic radiation.
 16. Anelement according to claim 1 wherein the polymeric binder comprises atleast one vinyl addition polymer containing free carboxylic acid groups.17. An element according to claim 1 wherein after imagewise exposure toactinic radiation and solvent development the nonimage areas have anoptical density not exceeding 0.3 in the 350 to 400 nm region.
 18. Anelement according to claim 1 containing a dispersed pigment in thephotopolymerizable layer.
 19. An element according to claim 10containing a dispersed pigment wherein after imagewise exposure toactinic radiation and solvent development the nonimage areas have anoptical density not exceeding 0.3 in the 350 to 400 nm region.
 20. Anelement according to claim 18 wherein the dispersed pigment is carbonblack.
 21. An element according to claim 12 wherein the actinicradiation absorber compound is a dye and/or pigment in an amount of 15to 40% by weight of the photopolymerizable layer.
 22. An elementaccording to claim 14 wherein the overcoat layer contains a mattingagent.
 23. An element according to claim 22 wherein the matting agent istaken from the group of colloidal silica, corn starch, rice starch, andmixtures thereof.
 24. An element according to claim 1 wherein theelement is substantially transparent in at least a portion of thevisible region of the spectrum.
 25. An element according to claim 24wherein the photopolymerizable layer transmits light in the magenta,cyan or yellow regions of the visible spectrum.